Photographic elements are coated on a wide variety of support materials, with the three major categories being plastics, paper and glass. Plastics are used as the support when a combination of transparency, strength, dimensional stability and light weight is needed. Useful plastics include cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate, polystyrene, polycarbonate and polyethylene terephthalate. Paper is the support of choice in situations where the physical property requirements are not too demanding, cost is a major factor or an opaque base is needed. Typically, photographic paper supports are coated on both sides with a thin layer of a polymeric resin such as polyethylene. Glass has the advantages of excellent dimensional stability and extreme flatness but is disadvantageous in that it is expensive, heavy and brittle. Photographic glass plates typically range in thickness from about 1 to about 10 millimeters and are graded by flatness. Those with the lowest degree of flatness are used in such applications as photomicrography and graphic arts, those with an intermediate degree of flatness in such applications as photofabrication, stereoplotters and aerial cameras, and those with the highest degree of flatness in such applications as high-precision stereoplotters, ballistic and aerotriangulation camera systems and special scientific investigations.
For ease of description, the photographic plates are hereinafter described as "glass plates." It should be understood however that rigid transparent plastic materials can be used as the support in place of glass. Examples of such materials include polymethylmethacrylate and polycarbonate. The use of glass is preferred where excellent dimensional stability is needed but the use of a rigid transparent plastic material such as polymethylmethacrylate or polycarbonate is preferred where machinability is important. Thus, unless the context requires otherwise, the term "glass plates" is used herein in a broad sense to also encompass the rigid transparent plastic materials.
The coating of glass plates with photographic layers is a very demanding art since the layers must be extremely thin, highly uniform in thickness, and completely free from defects. One of the most effective techniques for accomplishing such coating is that described in U.S. Pat. No. 4,033,290, issued Jul. 5, 1977, in which a coating system based on wicking action is employed. In this system, an absorption wick having an arcuate surface is utilized in combination with a transfer wick that is in conforming contact with the arcuate surface. The difficulties involved in coating glass plates are described in this patent as follows:
"The coating of photographic glass plates is a very exacting coating operation in view of the stringent requirements that such plates must be capable of meeting. Thus, for example, to be used successfully for the coating of photographic glass plates the coating process must provide (1) complete coverage of the area which is to be coated, i.e., freedom from "skips" which would result in uncoated areas of even miniscule dimensions, (2) a coating layer which is extremely thin, (3) exactly uniform wet coverage, (4) freedom from coated material on the edges of the plate, (5) freedom from streaks or other coating defects, and (6) freedom from contamination with dust or other foreign materials. In addition the process must be capable of handling fragile sheets of glass without damage."
While the method and apparatus of the aforesaid U.S. Pat. No. 4,033,290 represent an important advance in the art of coating photographic glass plates and have proved highly successful, there are still serious concerns associated with their use. For example, there is a continuing need to still further reduce the number of streaks and other coating defects that occur. It would also be very desirable to be able to coat at much higher speeds than those that are attainable with a wick coater. Moreover, since photographic glass plates are needed in a very wide range of different sizes, there are inherent limitations in any coating operation that make it difficult to meet such needs. Thus, for example, a single coating line is not well suited to coating a very wide range of plates of differing widths, but the provision of multiple coating lines in order to have a separate line for each width desired is economically impractical. Yet another critical problem involved in the manufacture of photographic glass plates is the difficulty in providing edge-to-edge uniformity in the thickness of the silver halide emulsion layer.
As an alternative to coating the functional layers directly on the glass, photographic glass plates have been produced by adhesively securing a conventional radiation-sensitive photographic film to a sheet of glass by means of an adhesive layer interposed between the glass and the film support. The adhesive layer is typically formed by coating a liquid adhesive composition on the surface of the glass plate. The product resulting from such a process comprises a glass support having, in order on one surface thereof, the adhesive layer, the film support material--for example acetate film base or polyester film base--and the silver halide emulsion layer, as well as any other desired functional layers. This method of producing photographic glass plates avoids the problems involved in coating photographic coating compositions on glass. However, photographic film base is typically relatively thick, e.g., in the range of from about 0.1 to about 0.3 millimeters, and may exhibit many minor imperfections and, as a consequence, the optical characteristics of such photographic plates can be seriously degraded if the film base has significant imperfections. Moreover, such a technique still requires coating of an adhesive composition on the glass plate surface with all the difficulties entailed in coating of fragile glass plates. Furthermore, it is exceedingly difficult to prevent excess liquid adhesive composition from coming into contact with the silver halide emulsion layer, whereby it swells the emulsion and seriously damages it.
A wide variety of laminate materials have been proposed for use in the photographic art and these laminates can be employed to protect the emulsion layer of photographic glass plates. Examples of such laminates include those disclosed in U.S. Pat. Nos. 4,077,830, 4,337,107, 4,378,392, 4,581,267 and 5,085,907. However, the photographic glass plates utilized in conjunction with such protective laminates have been made by conventional techniques as described hereinabove.
U.S. Pat. No. 2,591,665 describes a process of laminating photographic film to glass to provide enhanced dimensional stability. In the process described, a glass plate is coated with an adhesive, such as an adhesive of the liquid casein and rubber emulsion type, and united with the photographic film by the use of pressure rollers and the adhesive is then allowed to set so as to bond the film to the glass. This process is disadvantageous in requiring that the adhesive be coated on the glass, in requiring a difficult operating step in which fragile glass plates are passed through pressure rollers, and in requiring a prolonged period to set the adhesive. The '665 patent recognizes the problem of contamination of the silver halide emulsion layer with the liquid adhesive composition and proposes the use of an absorbent protective sheet such as a sheet of paper, to blot up excess adhesive. However, if such an absorbent sheet wrinkles during the lamination process, as is very likely to occur, the wrinkles will damage the silver halide emulsion layer.
Belgian Patent 802,920 describes a process for manufacture of photographic glass plates in which a glass plate support is coated with a wet gelatin layer, a silver halide emulsion layer located on a temporary support film is brought into contact with the wet gelatin layer and the temporary support film is stripped off. Such a process is very difficult to employ on a commercial scale because of the difficulty of providing an emulsion layer that will readily transfer to the coated glass support yet not release during coating, drying, winding and finishing operations involved in its application to the temporary support. The process is also disadvantageous in that it requires that the gelatin which is employed as an adhesive be coated on the glass support and any process in which glass is coated is difficult to utilize commercially because of its fragility.
U.S. Pat. No. 5,254,447 describes several techniques for manufacturing photographic glass plates which are highly advantageous in comparison with the prior technique of coating the functional layers on the glass. Thus, in the '447 patent the silver halide emulsion layer and any other required functional layers, such as antihalation and protective overcoat layers, are applied to the glass plate support by lamination. In a first embodiment of the invention of the '447 patent, a release layer, a protective overcoat layer, a silver halide emulsion layer and an antihalation layer are coated, in order, on a flexible polymeric film and laminated via an adhesive layer to a glass plate having suitable thickness and flatness for use as a photographic support. The release layer is designed to have greater adhesion to the flexible polymeric film than to the protective overcoat layer, whereby the film and release layer can be stripped off prior to the plate being put into use. Depending on the product requirements, the antihalation layer and/or the protective overcoat layer can be omitted. Also, depending on product requirements, there can be two or more silver halide emulsion layers and additional layers providing other functional characteristics can be included. In a second embodiment of the invention of the '447 patent, an element comprising a release sheet, a very thin polymeric film whose thickness is much less than that of the release sheet, and an adhesive layer interposed between the film and the release sheet, is prepared and utilized as a support material on which there is coated, in order, an antihalation layer, a silver halide emulsion layer and a protective overcoat layer. Again, in this form of the process, additional layers can be included and the antihalation layer and/or the protective overcoat layer can be omitted. To prepare a photographic glass plate utilizing this form of the process, the release sheet is stripped away and the remainder of the element is laminated via the adhesive layer to the glass plate.
The manufacturing techniques of the '447 patent employ pressure-sensitive or heat-activatable adhesives and require the use of release layers. This adds substantially to manufacturing costs. Moreover, it is difficult to get such adhesives to fully and uniformly coat the surface to which they are applied and removal of excess adhesive is particularly difficult. Heat-activatable and pressure-sensitive adhesives are prone to trapping particulates which can result in surface irregularities which hurt resolution of the photographic plate. Pressure-sensitive adhesives typically utilize organic solvents which require additional expense and effort for safe handling and for capture of solvent vapors. While the techniques of the '447 patent avoid the difficulties of coating the functional layers directly on the glass plate support and represent an important advance in the art of manufacturing photographic glass plates, further improvement by way of simplifying the elements and steps employed in the process would be highly advantageous in this art.
It is toward the objective of providing a greatly simplified, lower cost and more versatile procedure for manufacturing radiation-sensitive photographic plates, that does not require the coating of any layers on the rigid support, that the present invention is directed.